The catalysts, micro reaction chambers, molecular storage, and molecular sieves
often consist of solids with microscopic
cavities. The most important and
versatile class of porous materials is the family of silicates known as
zeolites. However, porous frameworks aren't found exclusively in inorganic
materials. Organic materials can also be crisscrossed by narrow channels. A team
of Canadian and Russian collaborators has now identified a new class of
"biozeolites" made of simple peptides.
For their experiments, Dmitry V. Soldatov, Igor L. Moudrakovski, and John A.
Ripmeester chose to use a very simple type of protein. They limited themselves
to two protein building blocks, the amino acids valine and alanine, and hooked
them together. The result is two different dipeptides, depending on which amino
and acid groups are coupled: alanyl-valine (AV) and valyl-alanine (VA). Both
crystallize as microporous solids.
The crystals consist of spiral dipeptide
chains, each with an open channel in the center. These little channels are not
straight, but twisted. What is unusual is that all of the channels are twisted
in the same direction, to the right. The image and mirror image, in this case
right- and left-turning spirals, are not identica.
Amino acids are also chiral, the naturally occurring form being the
"left" version-which leads to right-handed channels in the dipeptide crystals.
Materials with chiral channels are difficult to produce, but are highly
desirable because they are used for the often decidedly difficult separation of
the "left" and "right" versions of chiral molecules.
Although the AV and VA crystals have very similar structures and dimensions,
there are distinct differences: the noble gas xenon is held much more tightly by
VA channels than by the AV pores. The reason for this seems to be the slightly
smaller pore diameter of the VA crystals; the smaller cavities result in more
intensive interactions between the gas atoms and the pore walls.
If the diversity of possible small peptides is taken into consideration, many different porous materials couldt be produced: The type,
number, and order of the coupled amino acid building blocks could perhaps be
used to tailor the pore properties of these "biozeolites" for particular
applications. Nontoxic peptides would also be suitable for biomedical